Nuclear magnetic resonance (NMR) imaging is a technique for creating pictures of body structures or organs without the need of exploratory surgery. However, unlike X-ray examination, NMR does not use any ionizing radiation. In addition NMR procedures do not require the injection of radiactive substances which otherwise are utilized to provide for measurements of the substances after their introduction into the body. Instead, a large magnet, a radio transmitter/receiver and a computer are used to obtain information from certain atoms in the human tissue. This information in turn is then utilized to create images of pictures of the internal anatomy of the body.
In particular, NMR systems can electronically produce multislice images of a given volume in standard orthozonal sections or at any angle with respect to standard body axes. Well-known NMR devices today include, for example, the GYROSCAN which is manufactured by the Philips Medical Systems, Inc., 710 Bridgeport Avenue, Shelton, Conn. 06484. For further discussion of the operation of an NMR device, reference is made to U.S. Pat. No. 4,585,992 entitled NMR IMAGING METHODS and which was assigned to the Philips Medical Systems, Inc., Shelton, Conn. Accordingly, this patent is incorporated herein in its entirety. Another NMR resonance imaging system is the VISTA MR.TM. system which is manufactured by Picker International Inc., 595 Miner Road, Highland Heights, Ohio 44143.
There is almost no limit as to what portion of the human body can be imaged with an NMR imaging system such as either of the two mentioned above. Since the utilization of NMR devices is well-known in the field, further discussion of the operation of such devices is deemed unnecessary.
However in the course of utilizing NMR imaging systems, it is necessary occasionally to evaluate or otherwise calibrate the measurements typically obtained with an NMR device. Such calibration and evaluation is obtained by the use of what are termed phantoms which are structural bodies that are positioned in place of the patient within the NMR device and provide ready reference for geometrical calibration of structures already established in the phantom itself. Such phantoms provide for the determination of the accuracy and reproducibility of thickness, thickness uniformity and distance.
Phantoms for NMR illustrated and described, for example, in U.S. Pat. Nos. 4,280,047; 4,499,375; and 4,551,679. According to the latter patent, a phantom for an NMR machine is described as including a plurality of containers each of which is filled with a material having a known spin density (T1) or T2 characteristic that differs a preselected amount from the spin density (T1) or T2 characteristics of the material in at least another portion of the other containers. As illustrated in the U.S. Pat. No. 4,557,679, the phantom is employed in an NMR machine which is designed for use in medical diagnosis by measuring the spin density T1 and T2 of the human body along selected planes of slices through the body itself. The phantom which is provided to test the operation of characteristics of the NMR machine includes a plurality of containers of non-magnetic material such as glass and each of which contains a material having a known spin density T1 or T2 characteristic that differs a preselected amount from the spin density T1 or T2 characteristic of the material in the other containers. In the embodiment illustrated, three different phantoms of rods A, B, C are shown as being identical but for their different orientations and locations within a base member.
According to U.S. Pat. No. 4,499,375, a nuclear imaging phantom is disclosed which includes a closed case 12 containing a plurality of straight, parallel columns in the form of a plurality of sets of rods. The rods also can include a plurality of spheres which are mounted on the end of their respective posts.
According to U.S. Pat. No. 4,280,047, a phantom for radiology is described as being formed of a rectangular sealed container filled with liquid to simulate clinical scattering. In particular, the container includes a series of fixed parallel steps that present individual planar surfaces progressively spaced from the top and bottom walls. A plurality of identical groupings of discrete geometrical attentuation objects are arranged along the respective planar surfaces of steps 12.
Other commercially available phantoms include the STC (slice, thickness and contiguity) phantom provided by Nuclear Associates, 100 Voice Road, Carle Place, N.Y. This phantom consists of a cylinder filled with a solution of copper sulfate in water. It also contains a series of 90 disks of 2 mm. thickness. Each disk has a vane or vanes oriented in various positions so that each vane filled with the copper sulfate solution, results in an image. Nuclear Associates also produces a phantom which is termed the MRI multi-purpose phantom that includes a folded steep ramp section, a star pattern section, a pin pattern section, a hole pattern section, a flood section and a concentric conic section.
Yet another phantom is that provided by Charles W. Coffey, II, Ph.D of the University of Kentucky Medical Center. Dr. Coffey's phantom which is described in Medical Physics, July-August 1986 of Volume 13, No. 4 of the AAPM Annual Meeting Issue, Lexington, Ky., is a three-dimensional conic section of NMR image-producing material. More specifically, Dr. Coffey's phantom consists of a hollow conical region into which is inserted a conical member thereby leaving a relatively thin annular region common between the base member and the conic member itself.
Although the phantoms described above have offered the ability to calibrate and evaluate the various NMR devices, the difficulty with these phantoms is that signal uniformity measurement is relatively difficult and the signal to noise ratio is relatively low. In addition geometric distortions are not easily recognizable with the various configurations discussed above.
I have invented a phantom which overcomes the aforementioned limitations and problems with the phantoms of the prior art and particularly provides a relatively large signal to noise ratio which permits improved detection of geometrical distortions, angularities and slice to slice interference.